1. Technical Field
The present invention relates to semiconductor fabrication equipment, and more particularly, to a displacement interferometer system for measuring displacement of a wafer stage holding a wafer, and an exposer using the same.
2. Discussion of Related Art
Semiconductor devices are being developed quickly with the rapid development in the information and communication field and the popularization of information media such as computers. Semiconductor devices are also being developed functionally to meet the requirements of high speed operation and high capacitance. Research and development of the technology for fabricating semiconductor devices are conducted to maximize the integration density, reliability and operation speed thereof.
The technology for fabricating a semiconductor device includes a deposition process for forming a processing layer on a wafer; a photo-lithography process for forming a processed layer on the processing layer formed by the deposition process and patterning the processed layer; an etching process for etching the processing layer, using the processed layer formed by the photo-lithography process as a mask; an ion implantation process for implanting impurity ions, using the processed layer as an ion implantation mask; and various anneal processes.
For example, the photo-lithography process forms a photosensitive layer, such as photo-resist which is used as the mask during the etching process or ion implantation process, in a pattern to be realized on a semiconductor substrate. The photo-lithography process includes a photo-resist coating process, a soft bake process, an edge exposure process, a side rinse process, a hard bake process, an exposure process, and a development process.
The photo-lithography process is performed using semiconductor fabricating equipment such as a spinner and an exposer. Since the photo-lithography process is important and essential in determining critical dimensions of a semiconductor device during a semiconductor fabrication process, research and development of the photo-lithography process are actively in progress.
An exposer includes an exposure light source for generating a light of a short wavelength, such as ultraviolet and X-ray, to which the photo resist is exposed; a reticle for transferring the light of short wavelength, which is supplied by the exposure light source, to a predetermined pattern image; an optical system including an objective lens for transferring the light of short wavelength to the reticle and reducing and projecting the light of short wavelength transferred through the reticle; and a wafer stage for supporting, aligning and plane-moving the wafer so that the pattern image is reduced and projected at a corresponding position of the wafer.
The wafer stage moves the wafer positioned at the focal distance of the objective lens horizontally. For example, the wafer stage enables the wafer to be aligned and horizontally moved so that an alignment mark formed on the wafer corresponds to an alignment mark formed on the reticle. In a scanner exposer, when the pattern image formed on the reticle is scanned to be transferred to the wafer during the photo-lithography process, the wafer stage may move the wafer horizontally in one direction. Then, the reticle is parallel to the wafer stage by a reticle stage and is moved in the same direction or opposite direction. The reticle stage or the wafer stage is substantially linearly moved by a power system such as a servo motor, and a movement distance thereof is measured by an encoder. However, since the encoder has an error which is larger than the size of the pattern formed on the surface of the wafer, a displacement interferometer system with high accuracy for distance measurement is used, instead of the encoder.
A displacement interferometer system used in an exposer is described in detail in U.S. Pat. No. 6,912,054 entitled “Interferometric Stage System”. The displacement interferometric stage system includes a support structure being fixed with a predetermined degree of planarity from the ground, a stage being horizontally moved on the support structure, a reflector being positioned on the stage, and an interferometer monitoring the position and orientation of the stage moved on the support structure, using a measurement beam.
The stage is set so as to be moved on a two-dimensional plane while horizontally supporting a wafer on the support structure. For example, the stage is divided into an upper stage and a lower stage which are moved, along the x-axis and the y-axis of the Cartesian coordinate system. The stage is linearly moved in one direction on a linear motion (LM) guide and is controlled by a servo or stepping motor control system which is rotated by a source voltage applied from the outside and controls the linear motion.
The reflector enabling the interferometer to measure the movement position of the stage is positioned at both perpendicular edges of the stage. The reflector will be described as a movable mirror in the description of the interferometer.
The interferometer includes a light source for generating a laser beam of a predetermined wavelength; a beam splitter for dividing the laser beam generated from the light source into a reference laser beam and a plurality of measurement laser beams and processing these beams; a reference mirror fixed to the support structure, for reflecting the reference laser beam, divided and progressed by the beam splitter, to the beam splitter; a movable mirror positioned on a sidewall of the stage, corresponding to the reference mirror, for reflecting the measurement laser beams while being moved; a sighting mirror for allowing the measurement laser beams to be incident on the movable mirror and returning the measurement laser beams reflected in the movable mirror to the beam splitter; and a detector for measuring a movement distance of the movable mirror, using the coherence of the reference laser beam and measurement laser beams returned to the beam splitter by the sighting mirror. The laser beam is an electromagnetic wave having a single wavelength. The laser beam has a proper and uniform single wavelength according to material used for the light source and is amplified to a predetermined intensity.
Accordingly, since the laser beam does not diffuse in a three-dimensional space, its directionality is high. Further, since the laser beam is the electromagnetic wave of a single wavelength, it is widely used in general devices for measurement and display, using the interference effects. When the reference laser beam and measurement laser beams, which are separated from each other in the beam splitter and respectively reflected in the reference mirror and movable mirror, are returned to the beam splitter, these beams are superimposed to cause constructive interference or destructive interference depending on phase difference, thereby forming an interference fringe of a predetermined intensity. As is known, such an interference phenomenon is mathematically described by Formula 1, wherein I, I1, and I2 are respectively the intensity of the interference fringe, the intensity of the reference laser beam, and the intensity of the measurement laser beams, and δ is the relative phase difference between the reference laser beam and the measurement laser beams.I=I1+I2+2 √{square root over (I1I2)} cos δ  [Formula 1]
A change in the intensity of the interference fringe is caused by the phase difference (δ). Accordingly, when the number of moving interference fringes is measured by the detector, the position of the movable mirror is calculated by Formula 2.
                    X        =                              X            0                    +                      N            ⁢                                                  ⁢                          λ              2                                                          [                  Formula          ⁢                                          ⁢          2                ]            
In Formula 2, X indicates the displacement of the movable mirror in the x-axial direction, X0 indicates an initial position of the movable mirror in the x-axial direction, N indicates the number of interference fringes, and λ indicates the specific single wavelength of the laser beam. Accordingly, the conventional interferometer measures the movement distance of the movable mirror in the x-axial direction, by detecting X0 in the x-axial direction and the number of interference fringes. The interferometer also measures the movement distance of the movable mirror in the y-axial direction, by detecting Y0 in the y-axial direction and the number of interference fringes. To measure yaw and tilt of the stage on which the movable mirror is positioned, which are corresponding to the degree of freedom of the stage, the interferometer may be designed apart from the x-axial or y-axial direction. To measure the yaw and tilt of the stage, the interferometer includes an additional movable mirror, in which an azimuth and a tilt in the x- or y-axial direction are measured with respect to a reference portion in the z-axial direction of the stage.
Accordingly, the conventional displacement interferometric system includes the interferometer to measure the movement distance of the movable mirror, using the interference phenomenon of the laser beam of single wavelength, thereby measuring the movement distance of the stage horizontally moved on the support structure and measuring the yaw and tilt of the stage.
However, the conventional displacement interferometric system has the following problems.
First, when the sighting mirror is shaken or yawed on the support structure by the vibration of the stage being moved horizontally, the angle of incidence of the laser beam for measurement is changed, thereby reducing the intensity of the measurement laser beams which are incident and reflected on the movable mirror and which are detected in the detector through the sighting mirror. Furthermore, it is not easy to catch the change of the sighting mirror and to correct the change, thereby decreasing productivity.
Second, the change in the angle of incidence of the measurement laser beams is recognized depending on the intensity of the measurement laser beams which are detected in the detector. However, it takes a long time to accurately correct the angle of incidence of the measurement laser beams, thereby decreasing the productivity.
Third, the change in the angle of incidence of the measurement laser beams being incident by the sighting mirror cannot be measured in real-time. Furthermore, when measuring the overlay of the surface of a wafer after the exposure and patterning processes, an alignment failure of the wafer positioned on the stage is only indirectly caught. This causes a failure in the exposure process of a number of wafers, thereby decreasing the yield of production.